We use BV CCD images to investigate the reality of the color gradient within a King type globular cluster NGC 7089. Surface photometry shows that there is a strong radial color gradient in the central region of the cluster in the sense of bluer center with the amplitude of ~0.39 0.07 mag/ in (B - V). In the outer region of the cluster, however, the radial color gradient shows a reverse case, i.e., redder toward the center. (B - V) color profile which was derived from resolved stars in VGC 7089 field also shows a significant color gradient in the central region of the clusters, indicating that lights from the combination of red giant stars and blue horizontal branch stars cause the radial color gradient. Color gradient of the outer region of NGC 7089 may be due to the unresolved background of the cluster. Similar color gradients in the central area of clusters have been previously observed exserved exclusively in highly concentrated systems classified as post core collapse clusters. We caution, however, to confirm the reality of the color gradient from resolved stars, we need more accurate imaging data of the cluster with exceptional seeing condition because the effect of completeness correlates with local density of stars.

Archival long-slit spectra, covering the wavelength range 4050~5150, have been used to investigate the radial behavior of absorption line features (G4300, Fe4383, Ca4455, Fe44531, Fe4668, and H) of an eliptical galaxy NGC 5322. The heliocentric recession velocity of NGC 5322 has been derived as 188851kms-1. Metallic absorption lines of NGC 5322 show significant radial gradients through the major axis. The minor axis shows much smaller radial metal line gradients than the major axis. The minor axis shows much smaller radial metal line gradients than the major axis. The mean slopes of Fe line gradients to the major and minor axes of NGC 5322 were estimated as -0.4330.064 and -0.2420.096, respectively. Significant radial gradients of H absorption of NGC 5322 are also detected both on the major and minor axes. It is shown that the radial metallicity gradients in NGC 5322 are smaller than expected in a simple dissipative collapse model. Rather, dissipationless collapse, such as hierarchical merhing, could have contributed during the initial stage of the galaxy formation.

We report the discovery of 17 new RR Lyrae variables in the central region () of the globular cluster M53. There candidates were identified by using the Welch & Stetson (1993) technique and confirmed by checking individual light curves in both B and V bands. The color-magnitude diagram of the horizontal-branch stars in the central region is compared with that for stars observed in the outer region by Rey et al. (1998). Including the new data from this study, we estimate the fraction of c typeRR Lyrae variables, n(c)/n(ab + c), to be 0.43 which agrees well with the valuse of other Oosterhoff group II clusters.

We present the results of a model calculation of the stellar distribution in a UV and centered at 2175 corresponding to the well-known bump in the interstellar extinction curve. The stellar distribution model used here is based on the Bahcall-Soneira galaxy model (1980). The source code for model calculation was designed by Brosch (1991) and modified to investigate various designing factors for UV satellite payload. The model predicts UV stellar densities in different sky directions, and its results are compared with the TD-1 star counts for a number of sky regions. From this study, we can determine the field of view, size of optics, angular resolution, and number of stars in one orbit. There will provide the basic constrains in designing a satellite payload for UV observations.

Bow shock formation, in case the supersonic solar wind flow is hindered by the atmosphere of Mars, is investigated. The atoms newly ionized from the extensive neutral atmosphere of Mars are loaded to the solar wind. By the conservation of momentum, the solar wind velocity is decreased. Then the supersonic flow velocity drops to the subsonic flow velocity in front of Mars at certain region, which is called the bow shock. The location of Mars subsolar bow shock is highly varying in the range of 1.3 to 2.5 Rm. Martian bow shock location is estimated by one-dimensional flux tube equations reduced from full three-dimensional MHD equations. The variability of Mars bow shock location effected by the solar wind conditions is studied. It is evident that the solar wind dynamic pressure change is able to make the Mars bow shock location variable.

A Fourier Transform Spectrometer was used to study upper mesospheric thermodynamic by observing the hydroxyl(OH) emission. Rocket-born and satellited-born photometers place the peak emission near 87 km. The instrument was installed in February 1999 at King Sejong station (), Antarctica and has been in routine operation since then. An intensive operational effort has resulted in a substantial data between April and June, 1999. A harmonic analysis was carried out to examine information on the tidal characteristics. The measured amplitudes of the 12-hour oscillation are in the range of 2.4-3.7 K, which are in resonable agreement with theoretical model outputs. The harmonic analysis also revealed 8-hour oscillation which is not expected from the traditional theoretical studies. In addition, the observed 8-hour oscillations are apparent and sometimes dominate the temperature variation in the upper mesosphere.

As the spatial resolution of remote sensing satellites becomes higher, very accurate determination of the position of a LEO (Low Earth Orbit) satellite is demanding more than ever. Non-symmetric Earth gravity is the major perturbation force to LEO satellites. Since the orbit propagation is performed in the celestial frame while Earth gravity is defined in the terrestrial frame, it is required to convert the coordinates of the satellite from one to the other accurately. Unless the coordinate conversion between the two frames is performed accurately the orbit propagation calculates incorrect Earth gravitational force at a specific time instant, and hence, causes errors in orbit prediction. The coordinate conversion between the two frames involves precession, nutation, Earth rotation and polar motion. Among these factors, unpredictability and uncertainty of Earth rotation, called UTI-UTC, is the largest error source. In this paper, the effect of UTI-UTC on the accuracy of the LEO propagation is introduced, tested and analzed. Considering the maximum unpredictability of UTI-UTC, 0.9 seconds, the meaningful order of non-spherical Earth harmonic functions is derived.

The optimal initial inclination for minimizing the variation of the Local Time of Ascending Node(LTAN) during the three year mission of the KOMPSAT is investigated. At first, the analytical equation for the inclination change by the Sun is derived and the optimal initial inclination by analytical method is derived. Then the analytically derived optimal inclination is checked by the numerical orbit propagation with including all major perturbations. Four different cases of the initial orbital elements are used for monitoring the LTAN variation of the LTAN. Therefore, a new optimal initial inclination by numerical orbit propagation for the KOMPSAT is found. In addition, the variations of the mean and osculating semi-major axis are investigated with the different atmospheric density values. The mean eccentricity vs. argument of perigee diagram for the frozen orbit is obtained.

An absorbed heat-flux method for ground simulation of on-orbit thermal environment of satellite is addressed in this paper. For satellite ground test, high vacuum and extremely low temperature of deep space are achieved by space simulation chamber, while spatial environmental heating is simulated by employing the absorbed heat-flux method. The methodology is explained in detail with test requirement and setup implemented on a satellite. Developed heat-load control system is presented with an adjusted PID-control logic and the system schematic realized is shown. A practical and successful application of the heat simulation method to KOMPSAT(Korea Multi-purpose Satellite)thermal environmental test is demonstrated, finally.

In this paper, KOMPSAT satellite launch and deployment operations are discussed. The U.S. Taurus launch vehicle delivers KOMPSAT satellite into the mission orbit directly. Launch and deployment operations is monitored and controlled by several international ground stations including Korean Ground Station (KGS). After separation from launch vehicle, KOMPSAT spacecraft deploys solar array by on-board autonomous stored commands without ground inter-vention and stabilizes the satellite such that solar arrays point to the sun. Autonomous ground communication is designed for KOMPSAT for the early orbit ground contact. KOMPSAT space-craft has capability of handing contingency situation by on-board fault management design to retry deployment sequence.

Light curves of two near-contact binaries DO Cas and SW Lyn are analyzed to investigate their Roche configurations. We made new BVR observations for DO Cas and employed the published observation data for SW Lyn. Our four night observations for DO Cas were secured over 8 days so that the contamination by the intrinsic light variation is minimized. Based on the WD model, we found that DO Cas could be classified as a near-contact binary, while SW Lyn is better described by semi-detached configuration. Furthermore it is also possible to classify DO Cas as a contact binary. It appears that there are ambiguities in the class of near-contact configuration, and further observations and re-analysis are required to establish the near-contact nature of binary systems.

The light curves of the representative 6 contact binary stars observed by OGLE Project of searching for dark matter in our Galaxy have been analyzed by the method of the Wilson and Devinney Differential Correction to find photometric solutions. The orbital inclinations of these Devinney Differential Correction to find photometric solutions. The orbital inclinations of these binaries are in the range of which is lower than that of the solar neighborhood binaries. The Roche lobe filling factor of these binaries are distributed in large range of 0.12 - 0.90. Since absence of spectroscopic observations for these binaries we have found masses of the 6 binary systems based on the intersection between Kepler locus and locus derived from Vandenberg isochrones in the mass - luminosity plane. Then absolute dimensions and distances have been found by combining the masses and the photometric solutions. The distances of the 6 binary systems are distributed in the range of 1 kpc- 6 kpc. This distance range is the limiting range where the contact binaries which have period shorter than a day are visible. Most contact binaries discovered in the Baade window do not belong to the Galactic bulge.

We performed CCD photometric observations of W UMa type contact binary BV Dra during eight nights from May 1996 to June 1999 using 61cm telescope at Sobaeksan Optical Astronomy Observatory, and completed BV R light curves of the system. From our observations, we derived nine new times of minimum lights (five timings for primary eclipse, four for secondary) and determined new light elements with the times of minima observed since 1999. Our BV R light curves and Batten & Lu(1986)'s radial-velocity ones were simultaneously analyzed with contact mode (Mode 3) of Wilson-Devinney's binary model, and the photometric and spectroscopic solutions for BV Dra were solved. In the analysis, we derived the solutions of 1999 light curves with and without spots, respectively. As the results, asymmetry of light curves may be interpreted as produced by the existence of two spots; hot spot on the secondary and cool on the primary. Combining solutions of light curves and radial-velocity ones, absolute dimensions of BV Dra are , , , . In mass-radius diagram, the less massive and hotter primary component of BV Dra is near TAMS and the secondary is near ZAMS, which is very similar to the other W-type W UMa binaries.

Differential time-series observations of BL Camelopardalis classified as a double mode SX Phoenicis type variable were secured with a charge coupled device. The observed photometric data was reduced using the IRAF Package and the differential magnitudes were obtained through aperture photometry. The periods of BL Cam were analyzed with the Generalized Least-Square Method by Vanicek (1971) and the Fourier Decomposition Method. It was found that the first and second period of BL Cam were 0.0391 day respectively which lead the period ratio of P1/P0=0.81. This period ratio is much different from 0.78 determined by other investigators and also much more larger than that of other double-mode SX Phe type variables. In addition, this period ratio is much different from the value expected from the relation between the metallicity and period ratio. From these results, it can be confirmed that BL Cam is the most extreme case among all double-mode SX Phe type variables.

We calculate the theoretical line profiles in order to investigate the influence of various velocity parameters. Line profiles are calculated by using the exponential velocoty law with two acceleration regions for orbital phases = 0.07 and = 0.06. From this compttation we find that the influence of the wind velocity gradient on a giant star is more important in the region near the star than in the region away from the star. The observed lines show stronger emission than the calculated line profiles and we interpret the difference is caused by the inhomogeniety in the atmosphere of 32 Cyg.

A thrusters modulation method is suggested for spacecraft attitude control with asymmetric configured thrusters during orbit adjustment maneuvers. Attitude is controlled by thrusters off-modulation during orbit maneuvers. Usual control method for symmetric configured thrusters cam not be applied for asymmetric configured thrusters. Disturbance induced from thrusters asymmetricity shall be compensated during pulsing. In this paper, a thrusters control method using null solution is suggested, which is shown working well without inducing disturbance.

In order to develop star trackers for a satellite in our country we studies selection procedure of optical parameters. For logical selection of the optical parameters, we simulated the entire processes in which star lights imaged on a CCD sensor were read into and processed in an associated electronics. The simulation resulted in relations between star's magnitude and achievable pointing accuracy, from which we derived optimal optical parameters to satisfy a required pointing accuracy of a star tracker. The selected optical parameters were used in an optical system design of a star tracker with a pointing accuracy of 10 arcsec.

To determine a geographical position by GPS signal, the effect of the ionosphere must be considered to improve accuracy. This has led us to continuously try to find the TEC of the ionosphere by using the GPS signal. So far the way to find TEC has been developed and the information obtained from this can be used not only to increase the accuracy of determining the position, but also to study the ionosphere. In this research, the TEC MAP over Korea was obtained by using the data collected from eight GPS stations around the Far East Asia, which is the common way to represent TEC over some regional or global region.

Solar activities ejecting high energy particles influence satellites and satellite communications as well as perturb geomagnetic fields. To understand space environments near the Earth being influenced by the Sun, we must study about the magnetosphere, the ionosphere, and the atmosphere beforehand. To study this issue, we investigate some ionospheric models, atmospheric models and geomagnetic field models : IRI(International Reference Ionosphere), PIM(Parameterized Ionospheric Model) and IGRF(International Geomagnetic Reference Field). We develop the models and build a web site to serve IRI, PIM and IGRF model on the internet so that one can easily get information of daily and global ionospheric and geomagnetic variations.